System and method for draft safeguard

Abstract

A gas furnace responsive to a thermostat includes a thermal switch wired in series between the furnace power supply and the thermostat. A microprocessor connected to the thermal switch detects when the switch opens and closes, carrying out prearranged programs in response thereto. The thermal switch is mounted so that it opens when an over-pressure in the furnace draft system is detected, as evidenced by hot flue gasses passing over the thermal switch probe. The switch is allowed to cycle at least one time before the furnace is disabled. After a certain period of the time, the combustion cycle is reinitiated and the above steps are repeated if the thermal switch again resets.

1. In a gas furnace that is responsive to a thermostat, and which contains a power supply, a gas valve, and an autoresettable thermal switch having a thermal switch probe that is responsive to an over-pressure condition, and a microprocessor responsive to said thermostat and said thermal switch for controlling said power supply and said gas valve, and wherein said gas furnace includes a furnace vent system, a process for detecting a blockage in said furnace vent system, comprising the steps of:
 mounting said autoresettable thermal switch adjacent to an entrance to said furnace vent system so that said thermal switch opens due to a flue pressure exceeding a certain level and causing hot flue gasses to pass over said thermal switch probe;  
 electrically connecting said thermal switch in series between said power supply and said thermostat;  
 programming said microprocessor to carry out the following steps upon said thermostat calling for heat; 
 (a) instituting a combustion cycle;  
 (b) sensing a condition of said thermal switch and detecting when said thermal switch opens;  
 (c) determining, when said thermal switch is open, whether the number of openings of said thermal switch or the duration that said thermal switch remains open exceeds a preprogrammed criterion;  
 (d) disabling, when the number of openings of said thermal switch or the duration that said thermal switch remains open exceeds said preprogrammed criterion, said furnace and reinstituting said combustion cycle after said furnace has been disabled for a specified period of time; and  
 (e) allowing, when said thermal switch is open and said furnace is not in a disabled state, said thermal switch to reset and reinstituting said combustion cycle upon resetting said thermal switch.  
 
 
   
   
     2. A method according to  claim 1 , wherein step (c) determines whether said thermal switch remains open for a period that exceeds a first preprogrammed period of time, and wherein step (d) reinstitutes said combustion cycle after said furnace has been disabled for a second preprogrammed period of time. 
   
   
     3. A method according to  claim 2 , wherein said first preprogrammed period of time is zero seconds. 
   
   
     4. A method according to  claim 2 , wherein said first preprogrammed period of time is between about 1 and about 10 minutes. 
   
   
     5. A method according to  claim 4 , wherein said first preprogrammed period of time is about 3 minutes. 
   
   
     6. A method according to  claim 2 , wherein said second preprogrammed period of time is between about 1 hour and about 8 hours. 
   
   
     7. A method according to  claim 6 , wherein said second preprogrammed period of time is about 3 hours. 
   
   
     8. A method according to  claim 1 , wherein step (c) determines whether said limit thermal switch is opened more than a first preprogrammed number of times during a third preprogrammed period of time, and wherein step (d) reinstitutes said combustion cycle after said furnace has been disabled for a fourth preprogrammed period of time. 
   
   
     9. A method according to  claim 8 , wherein said first preprogrammed number of times is zero. 
   
   
     10. A method according to  claim 8 , wherein said first preprogrammed number of times ranges from 1 to 20. 
   
   
     11. A method according to  claim 10 , wherein said first preprogrammed number of times is 10. 
   
   
     12. A method according to  claim 8 , wherein said third preprogrammed period of time is between about 1 hour and about 24 hours. 
   
   
     13. A method according to  claim 12 , wherein said third preprogrammed period of time is one heating cycle. 
   
   
     14. A method according to  claim 8 , wherein said fourth preprogrammed period of time is between about 1 hour and about 8 hours. 
   
   
     15. A method according to  claim 14 , wherein said fourth preprogrammed period of time is about 3 hours. 
   
   
     16. A method according to  claim 1 , wherein step (c) determines whether said thermal switch remains open for a period that exceeds a fifth preprogrammed period of time and determines whether the number of times that said thermal switch is opened and remains open for a period greater than a fifth preprogrammed period of time exceeds a second preprogrammed number of times during a sixth preprogrammed period of time, and wherein step (d) reinstitutes said combustion cycle after said furnace has been disabled for a seventh preprogrammed period of time. 
   
   
     17. A method according to  claim 16 , wherein said fifth preprogrammed period of time is between about 1 and about 10 minutes. 
   
   
     18. A method according to  claim 17 , wherein said fifth preprogrammed period of time is about 3 minutes. 
   
   
     19. A method according to  claim 16 , wherein said second preprogrammed number of times ranges from 1 to 20. 
   
   
     20. A method according to  claim 19 , wherein said second preprogrammed number of times is 2. 
   
   
     21. A method according to  claim 16 , wherein said sixth preprogrammed period of time is between about 1 hour and about 24 hours. 
   
   
     22. A method according to  claim 21 , wherein said sixth preprogrammed period of time is one heating cycle. 
   
   
     23. A method according to  claim 16 , wherein said seventh preprogrammed period of time is between about 1 hour and about 8 hours. 
   
   
     24. A method according to  claim 23 , wherein said seventh preprogrammed period of time is about 3 hours. 
   
   
     25. A method according to  claim 1 , wherein said step of disabling the furnace includes the step of closing said gas valve.